Java Iterable interface[Algorithm]

Q：什么是Iterable接口？

A：只有一个方法，放回Iterator，如下：

public interface Iterable<Item>{     Iterator<Item> iterator();}

Q：什么是Iterator接口？

A：有两个方法：hasNext()和next()。第三个方法: remove()可选

Q：为什么要用数据结构Iterable?

A：Java支持优雅的客户代码，应用场景如下：

for (String s : stack)    System.out.println(s);

Iterator<String> i = stack.iterator(); while (i.hasNext()){     String s = i.next();     System.out.println(s);}

下面，以Stack类实现Iterable为例，介绍Iterable的实现方法：

/************************************************************************* *  Compilation:  javac Stack.java *  Execution:    java Stack < input.txt * *  A generic stack, implemented using a singly-linked list. *  Each stack element is of type Item. * *  This version uses a static nested class Node (to save 8 bytes per *  Node), whereas the version in the textbook uses a non-static nested *  class (for simplicity). *   *  % more tobe.txt  *  to be or not to - be - - that - - - is * *  % java Stack < tobe.txt *  to be not that or be (2 left on stack) * *************************************************************************/import java.util.Iterator;import java.util.NoSuchElementException;/** *  The <tt>Stack</tt> class represents a last-in-first-out (LIFO) stack of generic items. *  It supports the usual <em>push</em> and <em>pop</em> operations, along with methods *  for peeking at the top item, testing if the stack is empty, and iterating through *  the items in LIFO order. *  <p> *  This implementation uses a singly-linked list with a static nested class for *  linked-list nodes. See {@link LinkedStack} for the version from the *  textbook that uses a non-static nested class. *  The <em>push</em>, <em>pop</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em> *  operations all take constant time in the worst case. *  <p> *  For additional documentation, see <a href="/algs4/13stacks">Section 1.3</a> of *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * *  @author Robert Sedgewick *  @author Kevin Wayne */public class Stack<Item> implements Iterable<Item> {<span style="color:#ff0000;">//声明，实现Iterable接口</span>    private int N;                // size of the stack    private Node<Item> first;     // top of stack    // helper linked list class    private static class Node<Item> {        private Item item;        private Node<Item> next;    }    /**     * Initializes an empty stack.     */    public Stack() {        first = null;        N = 0;    }    /**     * Is this stack empty?     * @return true if this stack is empty; false otherwise     */    public boolean isEmpty() {        return first == null;    }    /**     * Returns the number of items in the stack.     * @return the number of items in the stack     */    public int size() {        return N;    }    /**     * Adds the item to this stack.     * @param item the item to add     */    public void push(Item item) {        Node<Item> oldfirst = first;        first = new Node<Item>();        first.item = item;        first.next = oldfirst;        N++;    }    /**     * Removes and returns the item most recently added to this stack.     * @return the item most recently added     * @throws java.util.NoSuchElementException if this stack is empty     */    public Item pop() {        if (isEmpty()) throw new NoSuchElementException("Stack underflow");        Item item = first.item;        // save item to return        first = first.next;            // delete first node        N--;        return item;                   // return the saved item    }    /**     * Returns (but does not remove) the item most recently added to this stack.     * @return the item most recently added to this stack     * @throws java.util.NoSuchElementException if this stack is empty     */    public Item peek() {        if (isEmpty()) throw new NoSuchElementException("Stack underflow");        return first.item;    }    /**     * Returns a string representation of this stack.     * @return the sequence of items in the stack in LIFO order, separated by spaces     */    public String toString() {        StringBuilder s = new StringBuilder();        for (Item item : this)            s.append(item + " ");        return s.toString();    }           /**     * Returns an iterator to this stack that iterates through the items in LIFO order.     * @return an iterator to this stack that iterates through the items in LIFO order.     */    public Iterator<Item> iterator() {<span style="color:#ff0000;">//实现iterator方法，返回Iterator</span>        return new ListIterator<Item>(first);    }    // an iterator, doesn't implement remove() since it's optional    private class ListIterator<Item> implements Iterator<Item> {<span style="color:#ff0000;">//实现Iterator</span>        private Node<Item> current;        public ListIterator(Node<Item> first) {            current = first;        }        public boolean hasNext()  { return current != null;                     }<span style="color:#ff0000;">//实现hasNext方法</span>        public void remove()      { throw new UnsupportedOperationException();  }//remove方法可选        public Item next() {<span style="color:#ff0000;">//实现next方法</span>            if (!hasNext()) throw new NoSuchElementException();            Item item = current.item;            current = current.next;             return item;        }    }    /**     * Unit tests the <tt>Stack</tt> data type.     */    public static void main(String[] args) {        Stack<String> s = new Stack<String>();        while (!StdIn.isEmpty()) {            String item = StdIn.readString();            if (!item.equals("-")) s.push(item);            else if (!s.isEmpty()) StdOut.print(s.pop() + " ");        }        StdOut.println("(" + s.size() + " left on stack)");    }}